DeCosta is a bigger reason why the Ravens are typically a perennial playoff contender. He will succeed wherever he works provided he also has a competent game day coach / staff.
Harbaugh will underachieve wherever he goes because he is the main problem with a very faulty game day staff. His teams have success in spite of his post-kickoff failings, because of the talent assembled at the DeCosta / Newsome levels.
And so I believe beyond a shadow of a doubt.
I don't think we've seen enough of Decosta to assume that he's going to keep a team in contention indefinitely. The Ravens had a lot of problem years from 2013-2017 and then EDC/Newsome hit a grand slam in 2018, but some of their drafts since have been up and down. I like a lot of the things that EDC does, but some of the things he does (like grossly misjudging the market for WRs, and generally having problems with drafting for impact at the position) is really problematic in today's game.
I do agree with you to an extent on Harbaugh. I don't think his style is great for maximizing the team's ceiling, and, while any HC needs some good fortune to continually make it to the super bowl, I kind of feel like he's a bit more reliant on luck than some other top head coaches. He's still a good head coach but he's not someone that's going to really push you over the top. He'll get you in contention and keep you there but past that? I'm not sure.